Hollow-core photonic crystal fibers (HC-PCFs) are known in the prior art. See FIGS. 1a and 1b which depict end views of known HC-PCFs. FIG. 1a is an end-view of a HC-PCF, reproduced from article in Science cited below while FIG. 1b is image reproduced from University of Bath Website. Note that the gas volume is very small (scales are on images) and that the structure of FIG. 1b has a large space-to-glass ratio. In neither case can the outer diameter of fiber be seen.
HC-PCFs have been shown to guide optical beams in the visible light regime via Bragg Reflection (BR) at a core/cladding interface along their structure, as opposed to conventional optical fibers that guide light via total internal reflection (TIR) along the core-cladding interface. This property enables the HC-PCF fiber to guide light down its core, whose refractive index can be less than that of the cladding, even a gas or vacuum. Owing to the guided-wave nature of these fibers, the hollow core of the fiber can be filled with a variety of low or high refractive-index states of matter, including gases, cryogenic liquids (LN2), or critical-point media (ethane, Freon 113). By using such fibers, long interaction lengths can be realized that can far exceed lengths typical of free-space, focused media. This enables nonlinear optical interactions to be observed for laser powers not previously possible in short-interaction-length geometries.
The emergence of the HC-PCF enables one to realize SRS with materials that can now be configured into long interaction lengths. Such a device can service new wavelength regions (UV through the IR) as well as power-scalable lasers and optical amplifiers. In addition, now-classic SRS interactions can now be made practical for a variety of applications, including wavelength-agile laser communication devices, remote sensors, IRCM sources, etc. Prior to this guided-wave structure (the HC-PCF), SRS using waveguide geometries were constrained to materials with a refractive index greater than that of the cladding material of the guide (typically, glass), thus limiting such devices to doped glass cores and high-index liquids in lightpipes, such as CS2-filled capillaries. These nonlinear media have several limitations including competing nonlinear interactions, such as self-phase modulation, self-focusing, photochemical degradation, Brillouin scattering, and optical breakdown. Gases typically have higher thresholds for these competing effects. With such gases as H2, D2, or CH4, one can nearly span the entire near-IR to mid-IR range and beyond (2 μm to >14 μm) using either several Raman shifts in a single gas, or cascaded Raman shifts through a SiO2 fiber, in series with different candidate gases or other non-linear media, combined in a single, or separate fluid-filled HC-PCF sections.
The approach disclosed herein enables relatively high wall-plug efficient sources and optical amplifiers to be realized (≈3%), which is a factor of 20 times greater than prior art approaches. In addition, the approach disclosed herein enables room-temperature systems to be fielded, which can be either pulsed or continuous-wave, thereby opening up new classes of devices to service myriad applications. The disclosed tunable system can be scaled to many watts of single-mode, diffraction-limited output, or high-gain beam and image amplifiers.
The prior art includes a variety of Raman lasers, typically using gases, in free-space configurations, which involve focusing of optical beams into relatively short cells, filled with gases. Since the interaction lengths are typically on the order of μm to mm, and the laser beam waist is on the order of several μm, the SRS threshold is on the order of MWatts of peak laser power. This follows, since the SRS threshold scales requires G=g·I·L≈20, where G is the SRS gain (where g is dependent on the specific gas or other states of matter), I is the laser intensity, and L is the interaction length. In applications where the disclosed Raman device is used as an amplifier, the gIL can be less than 20.
SRS has also been observed in guided-wave structures, such as conventional fibers (glass-based core and cladding). Given the long interaction lengths possible, SRS can be observed with relatively low laser powers and applications of low-noise, high-gain SRS amplifiers are typical in the telecom industry.
Research in the past has also involved liquid-filled capillary tubes, which can serve as a guided-wave structure, so long as the liquid (e.g., CS2) has a refractive index greater than the glass, as well as hydrogen-diffused glass fibers. These approaches are not very practical, owing to the competing nonlinear optical effects in glass and liquids (self-focusing, self-phase modulation, SBS, photochemical degradation, and laser-based optical breakdown). Also, the refractive index constraint (n≧1.5), severely limits the candidate liquids. In the gas-diffused case, the shelf-life, stability and diffusion-limited hydrogen density limits the utility of this approach to that of academic interest.
The emergence of HC-PCFs has enabled researchers to now employ gases (such as H2) as Raman active media, and a demonstration of Raman conversion using a pulsed laser at visible wavelengths to a Raman-shifted or Stoke's-shifted output, also in the visible, has been published at visible wavelengths. See F. Benabid, J. C. Knight, G. Antonopoulos, and P. St. J. Russell, “Stimulated Raman Scattering in Hydrogen-Filled Hollow-Core Photonic Crystal Fiber”, Science 298, 399 (2002). In another demonstration, the ability of HC-PCFs to enable high-peak power propagation of short pules over long HC-PCF lengths with minimal pulse duration was demonstrated at telecom wavelengths. Specifically, a Xe-filled, single-mode HC-PCF was used to demonstrate high-peak power pulsed laser propagation over long lengths while maintaining a high potential communication bandwidth. Dimitre Ouzounoiv, et al., “Researchers report a photonic-band gap fiber that can transmit megawatt pulse,” Science 301, 1702 (2003).
The approach disclosed herein employs the following embodiments, which are not believed to have been previously discussed in the literature: This disclosure employs a near-IR, tunable fiber laser as the pump, coupled with a gas-filled HC-PCF specially designed to have a bandpass of at least 300% (say, from 1 μm to >3 μm) to access the IR to mid-IR and long-IR wavelength regions of the spectrum, as well as the incorporation of cascaded HC-PCFs, each with different gases, and each with a sequentially longer wavelength photonic crystal bandpass. Optionally, the sequential HC-PCFs may be composed of different materials (e.g., a solid SiO2 core, followed by HC-PCFs using SiO2 and As2Se3) to enable low-optical-loss guiding at progressively longer wavelengths, or the sequential HC-PCFs may be composed of a mix of cascaded HC-PCFs, with different gasses and/or different solids of the type just mentioned. Finally, the approach disclosed herein employs a tunable fiber pump laser to realize tunability across most of the IR spectrum. In addition, these systems can be used as single beam or image amplifiers using single-mode or multi-mode HC-PCFs, respectively. As noted, none of these notions appear to be discussed in the literature.
The present invention is not obvious in view of the prior art. The prior art does, indeed, describe and demonstrate SRS in the visible light spectrum in a single section of a H2-filled HC-PCF, using a pulsed laser as a pump. However, there is no discussion or suggestion of which we are aware of how to configure fibers and/or fiber materials to realize a desired output in the IR. In the present disclosure we consider, in general, multiple Stoke's-shifted beams to arrive at the desired output wavelength range. More specifically, in the prior art, there is no discussion of variants (that are described in this invention) of fibers and nonlinear media to realize the desired output and its properties. This includes, as an example, a cascaded series of fiber sections, with each fiber section consisting of a different core material (a solid core in one case, and various gas fills and/or pressures, either in a single fiber section or in sequential sections), different photonic bandgap structures in each section, different core sizes, different photonic crystal matrices, different materials (e.g., SiO2, As2Se3), as well as combinations thereof. In addition, the prior art does not consider the case of a tunable pump laser (in our case, a tunable Yb-doped fiber), which can lead to a tunable IR output beam, with a “magnified tunable range”—that is, a 8% pump-laser wavelength tuning range can result in a 100% tunable Stokes-shifted output wavelength range. Finally, by frequency modulating the pump laser, a Raman-shifted output beam can result in a “magnified” FM modulation deviation. The prior art simply deals with a single Stoke's shifted output beam, using a fixed wavelength, pulsed pump laser.